1. Field of the Invention
The present invention provides improved apparatus for dispensing controlled amounts of liquid into receptacles, the apparatus including means for loading disposable pipette tips onto fluid loading/dispensing cylinders and maintaining a positive pressure load on the tips during introduction of the liquid into the pipette tips and dispensing of the liquid thereafter into the receptacles, thereby providing an effective liquid seal between the pipette tips and cylinders during the operation cycle of the apparatus.
2. Description of the Prior Art
U.S. Pat. No. 5,226,426, assigned to the assignee of the present invention, describes a positive displacement apparatus for introducing measured amounts of liquid into receptacles. A liquid supply comprises pistons and cylinders and means are provided to move the cylinders upwardly and downwardly with respect to the receptacles, the former movement drawing liquid into the cylinders, the latter movement expelling liquid from the cylinders. Permanently attached needles are provided to expel air trapped in the cylinders and project into the base of cylinders and project into either the liquid supply or the receptacles, depending upon which part of cycle the machine is operating on.
Although the '462 patent utilizes a fixed needle arrangement, other fluid dispensing system use fluid containing plastic pipette tips, the tips being joined to a metal dispensing cylinder by positioning a tray of pipette tips below the cylinders and then moving the tray upwards towards the coaligned cylinders by an actuator located on or beneath the tray supporting table until the tips are secured to the adjacent cylinder. This design causes a significant amount of force to be exerted up into the associated dispensing heads and into the supporting structure. This in turn necessitates that the structure be large to support the force, a typical structure being in the form of a large "C", the force being applied to the open part of the "C" causing the remaining closed part to be very large. In addition, to the inherent disadvantages of utilizing large structures, the fact that the initially loading force is eventually removed increasing, the likelihood that the original seal formed between the plastic pipette tip and the metal cylinder will be broken during the loading and dispensing operation being limits the usefulness of the prior art devices.
What is desired therefore is to provide a compact, relatively lightweight apparatus for securing pipette tips to their associated cylinders, a load being constantly applied to the pipette tips whereby an effective seal is continuously maintained during the entire operation cycle of the apparatus to avoid leaking and contamination of the fluid.